In recent years, as there is a reduction in the dependence on fossil fuels, research of electric automobiles is advancing, and in part electric automobiles are already being put into practical use. The electric motor, which is the power source of an electric automobile, is different from an internal combustion engine that operates by directly burning fossil fuel, and typically, because the characteristics of torque and rotation speed of the output shaft such as when generating maximum torque at startup are preferable for an automobile, it is not absolutely necessary for an electric automobile to have a transmission that is necessary in a typical automobile that uses an internal combustion engine as the drive source. However, even in the case of an electric automobile, by providing a transmission, it is possible to improve the accelerating performance and high-speed performance. More specifically, by providing a transmission, the relationship between the traveling speed and the acceleration of the vehicle is close to that of a gasoline engine that has a transmission in the power transmission line, and it is possible to make operation of the vehicle smooth. This point will be explained with reference to FIG. 5.
For example, when a power transmission apparatus having a large reduction ratio is provided between the output shaft of the electric motor, which is the drive source for an electric automobile, and the input section of a differential gear that is connected to the drive wheels, the relationship between the acceleration (G) and the traveling speed (km/h) of the electric automobile is as the left half section of the solid line “a” in FIG. 5 that is continuous with the chain line “b”. In other words, the acceleration performance at low speed is excellent, however, traveling at high speed is not possible. On the other hand, when a power transmission apparatus having a small reduction ratio is provided between the output shaft and the input section, the relationship is as the chain line “c” in FIG. 5 that is continuous with the right half section of the solid line “a”. In other words, high-speed travel is possible, however, acceleration performance at low speed is lost. However, by providing a transmission between the output shaft and the input section and changing the reduction ratio of the transmission according to the vehicle speed, it becomes possible to obtain the characteristic such as illustrated by the left half section and right half section of the solid line “a”. This characteristic is nearly the same as that of a typical gasoline engine having the same output as indicated by the dashed line “d”, and in regards to the acceleration performance and the high-speed performance, by providing a transmission, it can be seen that an electric automobile can obtain the same performance as that of a gasoline engine.
FIG. 6 illustrates construction disclosed in JP 2006-022879 (A) as an example of conventional construction of a drive apparatus for an electric automobile in which a transmission is provided between the output shaft of the electric motor and the input section of a differential gear that is connected to the drive wheels. This drive apparatus for an electric automobile transmits the rotation from the output shaft of the electric motor 1 to a rotation transmission apparatus 3 by way of a transmission 2, and rotates and drives a pair of left and right drive wheels. A pair of gear transmission mechanisms 6a, 6b having different reduction ratios are provided between a rotating shaft 4 on the driving side that is concentric with the output shaft of an electric motor 1 and a rotating shaft 5 on the driven side. By switching a pair of clutch mechanisms 7a, 7b, it is possible to set one of the gear transmission mechanisms 6a (6b) in a power transmittable state, and switch the reduction ratio between the rotating shaft 4 on the driving side and the rotating shaft 5 on the driven side in two steps, large and small.
In other words, one clutch mechanism 7a of the clutch mechanisms 7a, 7b is set to be controllable by an actuator, and the other clutch mechanism 7b is set as an overrunning clutch that is disconnected when the rotation speed is a fixed value or greater. With the one clutch mechanism 7a in a connected state, and the other clutch mechanism 7b disconnected (idling), the rotation torque from the rotating shaft 4 on the driving side is transmitted to the rotating shaft 5 on the driven side by way of the gear transmission mechanism 6a, which is one of the gear transmission mechanisms 6a, 6b, that has a small reduction ratio. On the other hand, when the one clutch mechanism 7a is disconnected, and the other clutch mechanism 7b is connected, the rotation torque from the rotating shaft 4 on the driving side is transmitted to the rotating shaft 5 on the driven side by way of the gear transmission mechanism 6b having a large reduction ratio. The rotation of the rotating shaft 5 on the driven side is transmitted to the input section of the differential gear 8 by the rotation transmission apparatus 3, and the output shafts 9a, 9b that support the pair of left and right drive wheels are rotated and driven.
In the case of this kind of conventional construction, a pair of gear transmission mechanisms 6a, 6b are provided between a rotating shaft 4 on the driving side and a rotating shaft 5 on the driven side that are parallel with each other and separated from each other in the radial direction, so there is a problem that the drive device for an electric automobile will become large. Moreover, one clutch mechanism 7a of the clutch mechanisms 7a, 7b comprises an actuator for switching the engaged/disengaged state thereof, so there is a possibility that the weight of the drive apparatus for an electric automobile will increase. On the other hand, in order to improve the convenience of an electric automobile, and in order to lengthen the distance travelled per one charge, it is essential that the drive apparatus for an electric automobile be made compact and lightweight, and that the amount of power consumed per distance travelled be reduced.
Technology is disclosed in JP 2010-090947 (A) and JP 2010-223298 (A) as technology for making the drive apparatus for an electric automobile compact, in which rotating shafts that are concentric with the pipe-shaped output shaft of an electric motor and connected to a transmission having different reduction ratios are provided on inner-diameter side and the outer-diameter side of the output shaft of the electric motor, and by way of a pair of clutches, one of either the rotating shaft on the inner-diameter side or the rotating shaft on the outer-diameter side is rotated and driven. However, even in the construction disclosed in this patent literature, a heavy actuator is necessary for switching the engaged/disengaged state of the clutches, and so from the aspect of reducing the weight of the drive apparatus for an electric automobile, there is still room for improvement.